It is interesting to note that there are three weird sisters. Although they are used to represent the supernatural in Macbeth, the number three has religious significance in several religions and even in the Wiccan culture.
The witches give Macbeth three titles - thane of Glamis, thane of Cawdor, and king.
Traditionally witches stir pots or cauldrons containing animal and human body parts. The witches do not disappoint. The expected impact adds to the dramatic tension in the play and contributes to the ultimate decision that the audience or reader must make - whether Macbeth is responsible for his own downfall or whether he is controlled by "forces" outside the realm. The very contents of the cauldron will be used as a "charm" to further their aims and
everyone shall share i'th'gains.
The witches create storms, hail, thunder, lightening, "fog and filthy air" and also sink ships, sail in a sieve, vanish without warning and generally create confusion and disorder. When upset, after a disagreement with a sailor's wife, one of the witches creates a windstorm and brags about the “pilot’s thumb,” or small bone, she has as a charm. This should strike fear into anyone at the suggestion of her carrying human parts around.
Values in the witches world are reversed from the norm as "fair is foul." Their chanting is hugely symbolic in Macbeth and foreshadows what will happen in Scotland once temptation and ambition get the better of Macbeth and Lady Macbeth.
Darkness represents evil and fear and Macbeth refers to the witches as "secret, black and midnight hags."
The witches are presumably female but their appearances deceive as they have beards, thus creating ambiguity. Not only do their words and actions cause confusion but even their appearance.
Hectate, the weird sisters' senior and mentor, uses her power to catch "a vap'rous drop...." and "distill'd by magic sleights" she will cause more confusion in Macbeth's thoughts to ensure he puts "his hopes" above all rational thought.
The apparitions are strong symbols which encourage Macbeth to proceed with his plans. They give him confidence and courage but also add to his paranoia and obsession as he sees a vision of Banquo's sons which definitely does not form part of his own plan for the future. It is relevant to note that it is Macbeth's own insistence that the witches provide answers to his questions which encourages the witches to show him the apparitions and make him irritated and contribute toward his irrational behavior as he feels his issues are not suitably resolved.
The witches, significantly, do not appear towards the end. It is Macbeth's confusion that adds to the realism of the play as otherwise a play with strong supernatural influences may lose some of its efficacy. The witches absence also adds to the conclusion that Macbeth himself is responsible for his own downfall.